Certain aliphatic-aromatic copolyesters, abbreviated herein as “AAPE”, are known to be biodegradable, i.e., undergoing fragmentation and microbial breakdown within a composting environment. These copolyesters, unfortunately, suffer from poor melt strength in comparison with other film resins like polyethylene, thereby making processing on conventional polymer processing equipment (e.g. to produce blown film, cast film, fiber, etc.) difficult. For example, low melt strength often results in more line breaks, instability, and lower throughput rates on processing equipment, which increases the cost of the final polymer article. This lack of processability has restricted the range of applications of AAPE copolyesters and makes a low cost, high melt strength AAPE particularly desirable.
Melt strength is related to the viscosity of the polymer, particularly at low shear rates. It is a measure of the stiffness or “elasticity” of the polymer in the molten state. Higher melt strength generally results in greater ease in processing film or other articles such as, for example, profiles, extrusion blow molded bottles, fibers, etc., and better mechanical properties for the manufactured articles. Melt strength can be measured in many different ways. For example, the low or “zero shear” viscosity of the polymer can be used as one indicator of melt strength. An alternative method is to measure the time it takes for the polymer to sag a given distance under its own weight. This latter approach gives a more intuitive method for gauging melt strength and also incorporates how cooling of the film can also play a role in increasing the melt strength. Yet a third method is to determine the melt index of the polymer, which is known to be inversely correlated to the melt strength or a polymer.
Blends of polyesters and vinyl acetate containing polymers have been reported. For example, U.S. Pat. No. 3,937,757 describes molding compositions based on polybutylene terephthalate blended with 5 to 50 weight % of polyolefins including ethylene vinyl acetate. The PBT can contain up to 20% of modifying acids or glycols. The molded articles made from blends have improved electrical tracking resistance. U.S. Pat. Nos. 5,225,490; 5,661,193; 5,817,721; 5,889,135; 6,018,004; 6,020,39; 6,303,677 disclose biodegradable polyesters useful for biodegradable moldings, adhesives, foams, and coatings, etc. U.S. Pat. Nos. 5,599,858; 5,580,911; 5,446,079; and 5,559,171 disclose binary and ternary blends containing cellulose esters and biodegradable polyesters. These blends are disclosed as useful for shaped articles including fibers, films, and shaped articles.
AAPE's and some of their blends often have inadequate melt strength and can be difficult to process at lower temperatures. For example, blends of cellulose esters and AAPE's, because of the higher melt temperature of the cellulose ester, require high processing temperatures which offsets any gains in melt strength. Further, blending AAPE's with other polymers to improve melt strength is often expensive and reduces the biodegradability of the polymer. Thus, blends and additives that increase the melt strength but significantly reduce the composting rates provide little benefit. It is, therefore, desirable to develop an inexpensive AAPE copolyester that retains satisfactory biodegradability while exhibiting improved melt strength.